Assessing the Rigour of Environmental DNA (eDNA) Monitoring in Terrestrial Zones

This review asserts that the accuracy of biodiversity data depends heavily on the physical material sampled rather than just the molecular method itself. Historically, mapping the range of terrestrial species was a labour-intensive exercise in patience, often relying on visual sightings or physical traps that elusive animals easily evaded.

The Mechanics of Environmental DNA (eDNA) Monitoring

The authors examined 245 published papers to evaluate how different materials capture genetic evidence. They categorise these into three distinct groups: abiotic substrates (such as soil, water, and air), biotic substrates (including invertebrate blood meals or plant surfaces), and direct-evidence substrates (like scat or footprints). While abiotic samples offer broad coverage, they are prone to dilution. Biotic samples, such as extracting DNA from a leech, can provide concentrated data but introduce biological biases. Direct evidence offers the highest certainty of presence but requires finding specific traces, negating some of the passive efficiency researchers seek.

Technical Contrast: Source vs Detection

A critical distinction exists between the biological source and the detected signal. The review introduces the concepts of 'spatial and temporal deviations' to quantify this gap. In traditional capture methods, the animal is physically present. In eDNA workflows, the target is merely a chemical shadow. A soil sample might test positive for a species that passed through weeks prior (temporal deviation) or for DNA transported by water runoff from kilometres away (spatial deviation). This disconnect creates a high potential for false positives, where data implies a resident population that does not exist. Conversely, false negatives occur when the chosen substrate—say, leaf litter—fails to retain the specific genetic markers of a species that is actually present.

The study suggests that researchers must align their substrate choice with specific monitoring goals. Soil may be adequate for regional presence-absence data, but it lacks the resolution required for pinpointing active behaviours. By defining these deviations, the authors provide a framework to mitigate the risk of overinterpreting molecular data. The field must move beyond simply detecting DNA to understanding the physics of how that DNA moves and degrades in the wild.